None
Not Applicable
Field of the Invention
Electric utility companies supply power which their or other generating systems produce. The power is commonly transmitted through a grid of electrical high voltage alternating (AC) three-phase power lines. Occasionally a generation, transmission or distribution facility experiences a fault in a power line, switching or operating equipment which may, for example, result in a short circuit or other equipment failure on the power line. Monitoring systems include sensing and system management equipment to isolate a problem or reroute the power transmission or distribution. These power management systems include circuit breakers or switches to effect this circuit isolation or rerouting, and the sensed fault or abnormality on a line or at a substation may cause a monitoring station to trip a circuit breaker, either rerouting or causing power interruption to a customer. Some faults, in particular high impedance faults, can occur when a power line falls onto a high impedance surface such as dry grass or an asphalt road, but the wire remains energized because the high impedance surface insulates the wire to prevent it from generating a short circuit efficient to cause the circuit to trip. Utility companies attempt to identify such faults and quickly shut down the affected line to remove the hazard of the live line on the ground.
A variety of fault sensors have been developed to detect and signal power line faults. These sensors are read in a variety of substation or central station distribution panels. The heart of the protective system operation, in the event of fault, is a battery system in such as a substation at the transmission or distribution facilities or at the central station or the generation point. Battery systems perform a critical role in these fault or emergency situations. When emergencies do occur, it is essential that the battery systems perform as designed or serious consequences result in the substations since the batteries supply the power necessary to trip or close switches and circuit breakers which effect the opening of circuits to isolate or reroute the power.
It should be understood that the heart of any transmission or distribution substation is the station battery. As one or more of the distribution system monitors, either at the generation point or some other point in the transmission system, detects abnormalities on a section of the system, corrective action is taken. This corrective action may be either manually implemented or automatically, based upon the inputs to a computerized control system. The corrective action in respect of the transmission system, is the shutting down of various components, including substations, and/or rerouting the power distribution. Since the AC power within the system supplies the process power to various of the monitors and operating equipment within the system, it stands to reason that in those situations where the system is experiencing short circuits or other faults, the system power cannot be relied upon to operate the switches and circuit breakers to either shut down or reroute power distribution. It is the station battery in these circumstances which directly provides the power necessary to reposition circuit breakers and other switches, thus should the battery system fail during a fault on the system, there may be no way of clearing the fault or short circuit leaving the system vulnerable to major burn down of the facilities and widespread blackouts. The huge fault currents occurring with hard short circuits can easily cause meltdown in transformers, distribution circuits and substation busses resulting in a major meltdowns or fires in a substation or along the distribution route, resulting in not only loss of facilities, but widespread blackouts and potential injury to personnel.
Because of the importance of good reliability of battery system, numerous maintenance programs are performed to evaluate the batteries and battery systems. Many of these are performed under static conditions such as by taking specific gravity readings, cell voltage measurements, and electrolyte level maintenance. In addition to these static tests, the voltage and the charge current to the station batteries is also monitored as an indicator of the system status. In their usual arrangements, the battery back-ups consist of a number of common lead-acid wet cells connected in series to provide the voltage and current necessary to operate the switches and circuit breakers. Such systems may include multiples of three or six cell packs each having voltages of 6 or 12 volts connected in series providing such as 120 volts and 120 to 150 ampere-hours of DC power. Such batteries exhibit some similarities to those conventionally installed in automobiles and are known to periodically exhibit short circuits or increased resistance within individual cells. As is also well known, such faults affect the ability of the battery containing one of these cells to provide the rated output when called upon, and may well cause the degradation of the remaining components of the battery.
Standby storage batteries are designed to deliver energy to a load over a relatively long period of time at a slowly declining voltage, in contrast with the short-duration, high discharge typically provided by automotive batteries. Each standby storage battery includes one or more chemical cells, with multiple cells being connected in series so that the overall voltage, measured across the battery terminals, is equal to the sum of the individual cell voltages. Individual batteries are further connected together in series to form a battery bank having the level of voltage for the particular station battery.
The voltage measured across the positive and negative terminal of a battery cell, is a characteristic of the chemistry for that cell. In lead-acid batteries, the voltage across the terminals for a single cell is about 2 volts, while in a nickel-cadmium cell, the voltage is about 1.2 volts. In each cell, positive and negative reactants are bound together into positive and negative plates. Plates of like polarity are attached to a rigid, metallic supporting strap, which is fitted with a terminal post for connection to external loads. The assemblies of positive and negative plates with their respective straps and terminal posts are suspended in a jar or similar container, containing an electrolyte, and the plates are separated so that no direct contact between them occurs. Contact between plates of dissimilar polarity would result in a short circuit, rapidly discharging the cell and rendering it ineffective. The containers containing the cells are closed with a cover however, the terminal posts protrude for the connection to the external load.
When an electrical load is connected to the terminals of the battery, a chemical reaction occurs between the electrolyte and the materials making up the battery plates to make an electrical current flow between the plates of opposite polarity and thus through the terminals and the load. The battery xe2x80x9cdischargesxe2x80x9d by providing the DC current to the load, as the flow of active materials in the electrolyte and the plates equalize. By connecting a battery charger to the terminals of the battery, and effectively causing a reversed current to flow through the cells, between the oppositely charged plates, the reverse chemical reaction occurs and the battery becomes recharged.
A storage battery, like any source of electrical energy, has an internal impedance, which includes resistive, inductive and capacitive components. As the battery discharges, the current produces a voltage drop across the internal resistance of the battery in accordance with Ohms law. This voltage drop causes the voltage across the battery terminals to be somewhat less than ideal, i.e., the expected voltage, and the voltage drop consequently diminish the ability of the battery to power the load. The internal resistance of a storage battery at the time of manufacture is made as low as possible to minimize the voltage dropped during battery discharge. Over the life of the battery however, the internal resistance will increase, at a rate determined by such factors as how many times the battery undergoes cycles of discharging and charging, the effects of continuous charging on the electrical conductivity of internal cell connections, and the temperature of the electrolyte. The internal resistance of any cell will eventually increase to a level where the voltage drop across it during discharge is so great that the battery can no longer deliver power at its rated capacity. In most cases, internal cell resistance will not cause serious problems until the battery is near the end of its useful life.
If the battery circuit opens, (an internal battery condition involving the plates), the battery will no longer be able to discharge power into the load, and will consequently be useless as a standby energy source. Moreover, flammable gases released during the charging reactions may accumulate within the battery jar, and may ignite when internal connections burn (as by ohmic heating), causing an explosion that may damage equipment and injure personnel. Since station batteries include several battery cells in series, an open in any individual cell renders the entire series useless and unable to provide current to a load.
For the above reasons, it is important to have continuing information on the cell performance of the several cells making up a standby battery and means for the continuation of the supply of battery current in the event of an open cell. In recognition of the importance of monitoring the standby battery, numerous techniques and inventions have been developed. As will be recognized in the following review of the prior art, the several techniques and inventions are increasingly more complex, and therefore more expensive, difficult to install and maintain and consequently potentially conflicting in effectiveness. Likewise, as presently known prior to my invention thereof, no comparable means are currently available to overcome an open cell in a serial battery system.
U.S. Pat. No, 4,968,943 to Russo discloses an open battery bank detector. Russo""s non-intrusive open battery bank detector senses an alternating current component of the DC trickle charge carried by one of a pair of cables connected between the battery charger and the bank of batteries. When the AC component reaches a threshold level, a sensor circuit trips a relay which activates an alarm.
U.S. Pat. No. 4,546,309 to Kang discloses an apparatus and a method for locating ground faults. The Kang device utilizes a low frequency current generator having a variable output, a Hall-effect current probe for detecting the low frequency current produced by the generator, a filter and an amplifier connected to the output of the Hall-effect current probe for identifying and amplifying the low frequency signal. A readout element is connected to the output of the amplifier to indicate the relative magnitude of the low frequency signal.
U.S. Pat. No. 4,697,134 to Burkum discloses an apparatus and a method for measuring battery condition. The Burkum device measures the impedance of secondary cells that form the battery. The impedance measurement is made at a frequency selected to be different from those frequencies otherwise present in the charger-load circuit. A first application of the testing device monitors the battery for a change in impedance that can signal a developing defect in one or more individual cells or intercell connections. In a second application, the testing device is utilized to compare the impedance of individual cells and electrical connections to locate faulty components. A digital measurement of the measured AC current at the selected frequency is supplied to a computer or a digital system. A digital version of the measured voltage across the battery at the selected frequency is also supplied to the computer. The computer divides the voltage by the current measurement and records or logs the resulting impedance value on a regular basis.
U.S. Pat. No. 5,214,385 to Gabriel discloses an apparatus and a method for utilizing a polarization voltage to determine charge state of a battery. The test signal is a continuous square wave signal having a frequency less and 3 Hz. The test signal alternates between a voltage adequate to charge the battery and a lower voltage. The charging voltage is retained for a time sufficient to allow a polarization voltage to develop across individual battery cells.
U.S. Pat. No. 5,281,920 to Wurst discloses an on-line battery impedance measurement device. The impedance of battery cells within a battery bank is measured by dividing the bank into at least two battery strings. A load current is imposed on one of the battery strings and the battery cell voltage is measured within that string.
U.S. Pat. No. 5,574,355 to McShane discloses a method and an apparatus for the detection and control of thermal runaway in a battery under charge. The circuit determines the internal resistance or impedance and conductance (admittance) of the battery during the charge cycle.
U.S. Pat. No. 5,969,625 to Russo which discloses method and apparatus for injecting and detecting an audio frequency current signal carried by the battery bus and detecting a voltage drop. The audio current is injected by means of current transformers and an oscillator, detected by means of comparators and operational amplifiers and a signal is generated to represent the voltage and current signals. A microprocessor system is utilized to monitor the float voltage of the battery to assess stability.
The present invention enables a significantly more direct and simplified monitoring of the battery cells to ensure that a sufficient charge is being maintained as well as monitoring the cells for internal short circuits and resistance buildups. Further, the present invention provides a warning of internal irregularities within the cell system and includes by-pass means around problematic cells should such irregularities occur in a period when the system is called upon to provide power for circuit breaker or switch operation, thereby providing improved system reliability even though internal battery problems have occurred.
The present invention provides a fail-safe adaptation to a standby battery system, particularly of the type utilized in an electric power distribution system as well as enhanced monitoring of the system status. In the present context, electric power distribution system includes the related aspects of power generation and power transmission, since these facets are viewed as but subsets of the distribution scenario in respect that as the power is generated, it is directed to some utilization, i.e., undergoes some phase of distribution, albeit, by being transported to various sites for redirected transmission or utilization. The present invention is embodied in an unusually uncomplicated adaptation to the station battery to provide vastly improved reliability of the standby battery by effectively providing by-passes for non-functioning cells in a standby battery system. The invention further provides means for monitoring the status and condition of the several cells of the standby battery in order to observe abnormalities at early stages to be able to take advantage of scheduled maintenance or to anticipate imminent failure.
By analogy, battery cells are connected in series like the links in a chain. If one link opens up, the chain fails and any load carried by it is dropped. So it is with a battery. It is composed of cells, analogous to chain links, and if one cell opens, the battery fails and any load supplied by it is dropped. The present invention includes strategically placed diodes in parallel with a predetermined number of cells of the battery to by-pass any open circuit in the battery connection. The diode is analogous to a steel cable connected in parallel with a selected number of successive links. If a link fails, the cable spanning the several links including the failed one takes up the slack occurring with the failed link, maintaining the carrying of the load on the chain until a repair may be affected. If a cell opens, the associated diode by-passes the small group of cells in parallel with a particular diode, and the load on the battery continues to receive current. The monitor associated with the by-pass of the present invention provides information on the balance (or imbalance) of the battery system by observing the deviation of voltages and currents from balanced (normal) conditions in respective sections of the battery system through a simple Wheatstone bridge style of approach. The inventive monitoring system avoids the generation, addition or superimposition, and detection of separate monitoring signals on the existing station battery as well as the expensive and complex electronics required.
The rationale of the present invention is applicable to any electrical facility/system which uses batteries as a means of back-up power. These include substations owned and operated by major process companies such as petrochemical plants, telephone company switching facilities, offshore oil platform power plants, power systems on vessels and large interruptible power supplies (UPS).